Acoustic disturbances produced at supersonic flight speed by a propulsion system's nacelle cowling surface, along with those from the aerodynamic boundary surfaces of the inlet's captured stream tube and the jet plume exhaust from the nozzle, all influence the perceived loudness of an aircraft's sonic boom. A traditionally-designed nacelle produces numerous shocks that ultimately coalesce into the vehicle's overall sonic boom footprint. The challenge in attenuating the strength of these shock features lays in the inherent difficulty of rerouting flow streamlines in a supersonic flowfield without producing a discrete disturbance.
Spillage is an inlet characteristic that contributes strongly to sonic boom strength. Spillage is excess flow that is unusable by the propulsion system and naturally diverted (‘spilled’) around the sides of the intake through the inlet compression field. In a typical design, spillage occurs through the terminal shock, the only physical mechanism that can do so in a typical inlet design. The more spillage required, due for instance to off-design engine operation, the stronger the inlet's terminal shock automatically becomes, and the more detrimental the influence on sonic boom. Because it is a shock, this feature is discrete, overlaying an impulse into the vehicle's acoustic field. And because of its discrete nature, an impulse feature is difficult to attenuate or cancel using other low sonic-boom design techniques.
The angling of the cowling surface in the stream-wise direction at both the intake and nozzle exit contribute to sonic boom strength as does cowl blistering or bulging used to fit the nacelle around engine protuberances such as a gearbox. Intake cowl angle and nacelle bulging create blockage features to oncoming supersonic flow that generate compression shocks. In addition, the cowling angle at the nozzle exit, along with the downstream surfaces of any cowl bulging, produce expansion fans that tend to readapt to the local flowfield through compression shocks.
Finally, in a typical design, the exhaust jet plume itself aggravates the local acoustic field by generating strong compression shock and expansion-reshock features along its shear surface through flow-angle mismatch with the nacelle cowling and mal-adaption of the exhaust outflow pressure to the exit area of the nozzle. Off-design engine operation further aggravates this flow-angle and pressure mismatch. These issues are illustrated in FIGS. 1-3 which depict a conventional supersonic jet engine.
FIG. 1 schematically illustrates a prior art supersonic jet engine 20 having an inlet arrangement 22 and a nozzle arrangement 24 configured for operation at a predetermined Mach speed. Inlet arrangement 22 includes a cowl 26 and a center body 28. Center body 28 is coaxially aligned with cowl 26. Cowl 26 includes a cowl lip 30 and center body 28 includes a compression surface 32 and an apex 34 (also referred to as a “leading edge”). Cowl lip 30 and compression surface 32 together define an inlet 36 which admits air to turbo machinery 37.
A protruding portion 38 (also known as a “spike”) of center body 28 extends forward of cowl lip 30 by a distance L1. A supersonic airflow (not shown) approaching prior art supersonic jet engine 20 will encounter protruding portion 38 prior to entering inlet 36. The supersonic flow will initially encounter apex 34 resulting in an initial shock (not shown) that will extend in a rearward direction at an oblique angle that corresponds to, among other factors, the Mach speed at which prior art supersonic jet engine 20 is traveling. Conventionally, it is desirable to give protruding portion 38 a length that will result in an initial shock that extends from apex 34 to cowl lip 30 when the aircraft is moving at a predetermined Mach speed (also known as a “design speed” or a “cruise speed”). The length of a protruding portion that causes the initial shock to extend from apex 34 to cowl lip 30 when the aircraft is moving at the predetermined Mach speed will be referred to herein as a “conventional spike length”.
Nozzle arrangement 24 includes a nozzle 40 having a trailing edge 42. Nozzle arrangement 24 further includes a plug body 44 having a surface 46. Trailing edge 42 and surface 46 define an outlet 48. Plug body 44 is configured to control the expansion of the exhaust gases (referred to herein as the “exhaust plume”) exhausted from turbo machinery 37 during operation of prior art supersonic jet engine 20. As the exhaust plume travels downstream along plug body 44, plug body 44 has a continually decreasing diameter which provides space to accommodate the expanding gases of the exhaust plume. The ability of plug body 44 to control the expansion of exhaust gases of the exhaust plume ends at a trailing end 50 of plug body 44. At a point downstream of trailing end 50, the exhaust gasses of the exhaust plume will become fully expanded.
As illustrated in FIG. 1, a protruding portion 52 of plug body 44 extends beyond trailing edge 42 of cowl 40 by a distance L2. As is known in the art, the length L2 is selected by engine designers to correspond with a point of intersection of Mach lines propagating off an internal surface of trailing edge 42 when the prior art supersonic jet engine 20 is operated at a power setting that corresponds with the predetermined Mach number. The length of a protruding portion that corresponds with the intersection point of the Mach lines propagating off of an internal surface of trailing edge 42 will be referred to herein as a “conventional plug body length”.
FIG. 2 illustrates a prior art supersonic jet engine 20 traveling at the predetermined Mach speed. As prior art supersonic jet engine 20 travels down range, a free stream 52 of air approaches protruding portion 38. A portion of free stream 52 has been illustrated in phantom lines as forming a stream tube 54. Stream tube 54 has a diameter that corresponds with a diameter at cowl lip 30 and has a length that corresponds with a discrete period of time of operation of turbo machinery 37. All of the air within stream tube 54 will have some interaction with inlet arrangement 22—a portion of air within stream tube 54 will enter inlet 36 and the remaining portion of air will be spilled out of inlet 36.
Interaction between free stream 52 and apex 34 gives rise to initial shock 56. Interaction of free stream 52 with cowl lip 30 gives rise to a terminal shock 58 that propagates inwardly towards compression surface 32. Interaction of free stream 52 with cowl lip 30 also gives rise to a cowl shock 60 that propagates outwardly from prior art supersonic jet engine 20. The strength of cowl shock 60 corresponds, in part, with the angle at which cowl lip 30 is canted with respect to the horizon. The greater the angle, the stronger will be cowl shock 60.
Prior art supersonic jet engine 20 is configured to consume air at a predetermined mass flow rate while traveling down range at the predetermined Mach speed. As supersonic jet engine 20 moves down range, it will consume a smaller volume of air than is available in stream tube 54. Accordingly, a portion of the air within stream tube 54 will enter inlet 36 and a portion of the air within stream tube 54 will be spilled (“excess air”). The excess air within stream tube 54 must move in a direction that is radially outward with respect to inlet 36 in order to spill. However, the excess air cannot move out of the way of the approaching inlet 36 until after the excess air has passed through terminal shock 58. This is because the pressure disturbances arising out of the movement of the jet engine through the air towards stream tube 54 move only at the speed of sound while the jet engine approaches stream tube 54 at speeds in excess of the speed of sound. Thus, the first opportunity for the excess air to move out of the way of inlet 36 does not occur until after the excess air has passed through terminal shock 58. This phenomenon is illustrated in FIG. 3
FIG. 3 illustrates an outer layer 62 of stream tube 54 as it approaches inlet 36. Outer layer 62 represents the excess air, i.e., the portion of stream tube 54 that will not be consumed by turbo machinery 37 (See FIG. 2) and therefore will not enter inlet 36. Once outer layer 62 passes through terminal shock 58, it encounters the pressure disturbances associated with movement of prior art jet engine 20 through free stream 52. Outer layer 62 is then pushed laterally aside and overflows around cowl lip 30 as illustrated. This spilling of outer layer 62 out of the path of inlet 36 and around cowl lip 30 causes cowl shock 60 to move forward of cowl lip 30, thereby increasing its strength. The stronger this shock is, the greater will be the noise disturbance associated with it.
Returning to FIG. 2, an exhaust plume 63 is emitted from outlet 48. In the illustrated example, exhaust plume 63 comprises a straight cylinder of exhaust gas moving downstream away from nozzle arrangement 24. A free stream of air 64 approaching trailing edge 42 of nozzle 40 is traveling at an angle with respect to the straight cylinder formed by exhaust plume 63. As free stream of air 64 passes trailing edge 42 and encounters exhaust plume 63, the shear layer created by exhaust plume 63 behaves like a solid surface and causes free stream of air 64 to abruptly change direction. This abrupt change of direction gives rise to a tail shock 66. The encounter between free stream of air 64 and exhaust plume 63 may cause the gases of exhaust plume 63 to also abruptly change direction, causing the plume to generate additional shocks downstream (not shown). The strength of tail shock 66 (and the additional shocks in the plume) will depend upon the amount of misalignment between free stream 64 and exhaust plume 63.
As exhaust plume 63 passes downstream of trailing end 50, exhaust plume 63 will quickly reach a fully expanded condition. Starting from the point where exhaust plume 63 is fully expanded and moving downstream, exhaust plume 63 and free stream 64 will flow parallel to one another and both will flow in a direction that is parallel to a longitudinal axis of plug body 44. The transitional region, which starts where free stream 64 initially encounters exhaust plume 63 and which ends where exhaust plume 63 and free stream 64 flow parallel to a longitudinal axis of plug body 44, can give rise to expansions and compressions that, due to their proximity to tail shock 66, may contribute to the perceived loudness of sonic boom resulting from movement of prior art supersonic jet engine 20 at the predetermined Mach speed.
Accordingly, it is desirable to provide an inlet arrangement that is configured to mitigate the concerns described above. In addition, it is desirable to provide a method for assembling such an inlet arrangement. Furthermore, other desirable features and characteristics will become apparent from the subsequent summary and detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.